LCDs and LCOS display devices have a slow response time when switching from black to white or vice-versa, or from one grey level to another. This leads to a defocusing on moving objects, also known as smearing, as in conventional illumination of LCD panels (hold type) a lamp is always emitting light, also during switching of the modulator elements.
Jun-ichi Hirakata et al. describe in the article “Super-TFT-LCD for moving picture images with the blink backlight system”, SID 01 Digest, p.990–993, a method to improve moving picture image quality by alternating light on and light off periods (impulsive type emission), and by synchronising those periods with switching periods of the modulator elements. As shown in FIG. 1, a LCD is only illuminated by a blinking backlight during illumination periods 2, i.e. when the LCD is almost switched to the desired value. The pulsating backlight illumination has to be synchronised to the LCD refresh rate (=1/frame time, i.e. typically 60 Hz). The illumination duty cycle (ratio of pulse duration D to the pulse period P) and the phase difference (shifting of the pulse periods with respect to the LCD refresh rate) can be optimised to obtain the lowest smearing at a lowest light loss. The best results are obtained when the scanning of the LCD is taken into account. The rows of an LCD are driven one by one, i.e. the top rows of the LCD start switching at the beginning of a frame, while the bottom rows of the LCD are switched at the end of the frame time, as shown in FIG. 2. Ideally, the phase of the pulsating illumination has to change from top to bottom of the LCD panel. In other words, the illumination should be scrolling over the panel with the same speed as the driving scan.
SEOS Displays Ltd. has demonstrated putting an extra modulator 4, e.g. a fast switching ferroelectric LCD, between each of the three LCDs 6 and the projection lens 8 in case of a three-modulator projector, as shown in FIG. 3A, or one switching modulator 4 in front of the projection lens 8, as shown in FIG. 3B. Such a modulator 4 allows light to pass during a part of the frame period only. This solution with the extra modulator(s) 4 gives similar results as the pulsed illumination solution discussed above, but it has some drawbacks. If the modulator 4 is not patterned, then a good matching over the complete image is not possible, because the LCD scan is not taken into account. Ideally, the modulator 4 should be driven from top to bottom, but this leads to a more complex driving circuit and a loss of light, because of the necessary patterning of the modulator 4, which makes the aperture ratio decrease. If the modulator 4 has to be close to focus, then three modulators are needed, one for every colour, as shown in FIG. 3A. If only one modulator 4 is used, as in FIG. 3B, then it can only be inserted after the colour recombination system 10, and therefor it is less focused, and thus not well matched with the scanning of the LCD or LCOS panel 6. Furthermore, the modulator 4 has to be flat and fast. In practice this is realised with a fast switching LCD plus an extra polariser, which leads to extra loss of light, on top of the duty cycle loss.
A sequential colour display system for creating a full colour image projected onto an image plane is known from EP-1098536. A dynamic filter such as a colour wheel generates a series of primary coloured light beams that are swept across the surface of a spatial light modulator. Light rejected by the dynamic filter is recycled and is reapplied to the dynamic filter. Typically all three primary colours are produced simultaneously by the dynamic filter. The illuminated portion of the dynamic filter is imaged onto the modulator and a controller provides appropriate image data for each portion of the modulator in synchronisation with the sweep of the primary colour bands across the modulator surface. The primary colour bands are modulated by the spatial light modulator and the modulated light is focused onto the image plane. The viewer integrates the light arriving at each portion of the image plane over a frame period to provide the perception of a full-colour image.
It is a disadvantage that e.g. LCD spatial light modulators used in the display system described above, have a polariser in front of it and an analyser behind it. When randomly polarised light passes through the front polariser it becomes linearly polarised. This means that light having one polarisation direction is transmitted by the polariser, and that light having the other polarisation direction is absorbed. The polariser thus absorbs a lot of heat, and thus the spatial light modulator suffers from a heat problem. Heating of the LCD panel leads to images with wrong colours or grey values.